Function par_execute_sequential 

pub fn par_execute_sequential<T: Send>(
    tasks: Vec<Box<dyn FnOnce() -> T + Send>>,
) -> Vec<T>

Execute tasks sequentially, left-to-right. Used by sequential mode. Valid per the language spec (the fully-sequential interleave is always permitted). Available on every target — no threading required.

Signature matches par_execute (with Send bounds) so call sites can swap implementations without reshaping task boxes.